How to combine many faces into single face
If you want to use only 6 faces, then any detail on the faces will have to be represented using textures.  Fortunately, textures can be used to create several kinds of detail and I can think of the following off the top of my head being easy to do in webgl:


color
specular parameters (although you might have to use glsl)
normal (affects specular and other kinds of lighting)


Blender also supports


displacement
mirror parameters


You probably want to research how to BAKE a texture from your high-res model into something usable in a 6-quad model.  A quick google search comes up with 
https://www.youtube.com/watch?v=Tj-S5QAac3U about textures (which probably isn't useful in your case) and http://www.katsbits.com/tutorials/blender/baking-normal-maps-from-models.php which is for an ancient version of blender, but actually talks about baking from high-poly to low-poly.